This invention relates generally to a power control arrangement for resistive heating elements and more particularly to a power control arrangement for resistive heating elements used as surface units in cooking appliances such as ranges. It is an improvement to the power control arrangement disclosed and claimed in commonly assigned U.S. Pat. No. 4,443,690 to Thomas R. Payne and Alfred L. Baker, which makes use of a heater energy counter which is the subject of commonly assigned U.S. Pat. No. 4,551,618 to Payne.
In the arrangement disclosed in U.S. Pat. No. 4,551,618 a software implementable heater energy counter provides approximate heating element temperature information for a resistive heating element selectively energizable at various power levels according to the user selected power setting. The heater energy counter is incremented and decremented at rates which are approximately proportional to the rates of increase or decrease respectively of the temperature of the heating element in response to changes in the power setting to higher or lower power settings respectively such that the count of the heater energy counter approximately tracks the heating element temperature. In U.S. Pat. No. 4,443,690 heater energy counter information is used to initiate transient fast heat, fast cool operating modes which reduce the time required for the heating element temperature to respond to changes in the selected power setting for the heating element.
In the fast heat and fast cool operating modes the heating element operates at a power level higher than the selected power level for a predetermined time period when a change in power setting is detected, the newly selected power setting represents an increase above a predetermined relatively low power setting, and the count of the heater energy counter is less than a predetermined minimum value. The predetermined minimum power setting and predetermined minimum heater energy count are employed to prevent overshooting the desired temperature. Similarly, the fast cool operating mode is initiated when a change to a lower power setting is detected, the newly selected power setting is less than a predetermined relatively high power setting, and the count of the heater energy count is greater than a predetermined count to prevent undershooting the desired temperature. This arrangement does significantly decrease response time over a conventional open loop arrangement under those conditions which trigger operation in the transient mode. Since once initiated, operation in either of the transient modes is for predetermined times independent of the actual operating conditions, the system is relatively imprecise. Consequently, the threshold values for power settings and the predetermined time periods must be conservatively chosen to prevent overshooting or undershooting the target temperature. As a result, the time of response is enhanced primarily in the relatively extreme operating conditions of changing from very low to very high power settings and vice versa. Therefore, there is a need for a control arrangement using the heater energy counter which provides more accurate control enabling a more rapid response to changes in power setting throughout the range of power settings.
Accordingly, it is a principal object of the present invention to provide a power control system for household cooking appliances employing resistive heating elements which is an improvement to the power control system disclosed in U.S. Pat. Nos. 4,443,690 and 4,551,618, which further reduces the time required for the heating element temperature to reach the new operating temperature in response to changes in the selected power setting for the heating element throughout the range of operating conditions.